Esposito G. DVM, PhD1
Raffrenato E. BSc, MSc, PhD2
1. Department of Production Animal Studies, Faculty of Veterinary Sciences, University of Pretoria
2. Department of Animal Sciences, Faculty of AgriSciences, Stellenbosch University
Email: email@example.com; firstname.lastname@example.org
The Transition period involves multifactorial events that often lead to a status of negative energy balance and impaired fertility in dairy cows. To improve reproductive functions while maintaining high productivity, nutritional strategies are needed that provide regulatory signals to stimulate reproductive processes without compromising the partitioning of energy into milk production. These nutritional strategies will likely vary with cow genotype, dairy production system (e.g., pasture vs. TMR), management system, and their interactions. However energy balance seems to be the most important factors affecting the fertility of transition dairy cows. The control of the body condition score relative to lactation stage, milk yield, nutrition and health status, throughout the lactation cycle is perhaps the most practical way to quickly evaluate the energy balance of an animal and its feed intake. In addition, management strategies such as grouping, utilization of high palatable food and control of energy and protein concentration of the diet can play an important role to smoothly overcome the transition period. This paper wants to provide useful guidelines in the nutritional management of transition dairy cows on the base of the most recent research findings.
The decline in fertility in the modern dairy cow probably results from a historical emphasis on selection for milk yield 5. This low fertility is related to a multitude of health, physiological, and managerial factors, themselves a result of modifications to a number of physiological processes. These modifications presumably exist because they offer some advantage to milk production, but they may also prove detrimental to reproductive performance. Many of these issues have arisen because high-producing dairy cow utilize greater proportion of available nutrients for milk production at the expenses of body reserves and reproduction 13. Consequently, even when nutrient intake is increased to high levels, the outcome is simply an increase in milk production without necessarily an improvement in reproductive performance 28. To improve reproductive function while maintaining high productivity, nutritional strategies are needed that provide regulatory signals to stimulate reproductive processes without compromising the partitioning of energy into milk production. These nutritional strategies will likely vary with cow genotype, dairy production system (e.g., pasture vs. TMR), management system, and their interactions. This paper wants to provide useful guidelines for a better management of the transition period. The guidelines have been subdivided into the following key areas:
1. Body condition score;
2. Management of feed intake;
3. Grouping strategies: far-off and close-up diets;
4. Starch and protein;
5. Fatty acids.
Body condition score (BCS)
The loss in body condition (BCS) is one of the most visible signs of negative energy balance (NEB) in transition dairy cows. Generally, low BCS at any time in early lactation is associated with delayed ovarian activity, infrequent LH pulses, poor follicular response to gonadotropins, and reduced functional competence of the follicle 17. A period of postpartum anestrus is normal in the dairy cow; however, when it extends into the breeding season it could result in an increase in reproductive failure. In several studies 7, 47, 55 it was observed that the decrease of one point BCS at the moment of post-calving first insemination, can reduce cow reproductive efficiency by 17-38%. Consequently interval from calving to conception can increase over 120-130 days .Furthermore, Hayirli and collaborators25 demonstrated that dairy cattle that were over-conditioned (BCS > 4.0; using a scale of 1–5) in the last 3 weeks of gestation had a much greater depression in feed intake in the period immediately pre-calving when compared to cows with lower BCS. Those authors also reported a linear reduction in feed intake in the immediate pre-calving period as BCS increased. The maintenance of an optimal body condition score relative to lactation stage, milk yield, nutrition and health status, throughout the lactation cycle is perhaps the most important aspect of dairy cow management that facilitates a healthy transition from pregnancy to lactation. it can be postulated that an “ideal BCS profile” for dairy cows would dramatically reduce the negative impact of NEB on reproduction while still allowing cows to achieve high milk production (Figure 1). Loss in BCS is much greater in grazing cows receiving little or no supplementary nonstructural carbohydrates than in TMR-fed cows, probably because of differences in the time taken to recouple the somatotropic axis between the 2 production systems 43 and due to differences in blood concentration of insulin, a potent lipogenic agent 33.
Figure 1. Ideal BCS for dairy cows to minimize the effects of NEB on reproductive failure 11. The BCS is presented in the 5- point scale
Management of feed intake
The variation among cows in the severity and duration of NEBAL is primarily related to differences in dry matter intake and its rate of increase during early lactation 54, 49. Cows with decreased appetite will develop more severe NEBAL than cows of moderate conditioning and consequently will undergo increased mobilization of body fat and accumulate more triglycerides in the liver 45. Elevated liver triglycerides, in turn, are associated with a longer interval to first ovulation and reduced fertility 45, 9, 30. Maintaining feed intake during the transition period is the key to reduce NEBAL and avoid metabolic problems that are deleterious to performance. One of the most important aspects of ensuring adequate feed intake in the 3 weeks immediately prior to calving is to avoid the over-conditioning of dry cows 25. Cows overconditioned at calving will have decreased appetite and develop more severe NEBAL than cows of moderate conditioning. Hayirli and collaborators 25 reported a linear decrease of DMI with increasing BCS during the last 3 weeks before parturition. For farmers using grass silage as their conserved forage, large differences in voluntary feed intake potential should be recognized and only those silages with a high intake potential should be used where avoiding feed restriction is critical 50.
For grazing dairy cattle, it has been reported that once post-grazing sward height is less than 7 cm feed intake will be compromised 21. Kolver and Muller 33 indicated that feed DM intake is higher, and the degree of negative energy balance experienced in early lactation is lower, for TMR fed cows in comparison to grazing cows. The use of total mixed ration also facilitates the use of palatable feedstuffs such as molasses, which have been shown to improve energy balance in transition cows 48. Horan et al. 27, 28 reported higher levels of BCS loss in early lactation for cows fed pasture with low levels of concentrate supplement (up to 0.85 units of BCS), versus cows fed pasture with higher levels of concentrate supplementation.
This is supported by Mulligan end coworkers 39 and Kennedy and collaborators 32 who observed an increase in feed intake as concentrate supplementation rate increased at pasture. Furthermore it is important to consider the general health status of the cows since it was demonstrated that inflammatory conditions such as mastitis and endometritis, may reduce feed intake 29. Moreover, Goff 23 has extrapolated that the energy cost of an inflammatory response for a dairy cow of 600 kg body weight may amount to 4 Mcal per day, and that this has deleterious consequences for cows already in negative energy balance.
Grouping strategies: far-off and close-up diets
Many aspects of nutritional management (e.g., mineral nutrition beyond meeting basic nutrient requirements) appear to be much less important during the far-off period compared to the close-up period. However, numerous studies 2, 15, 26 in which cows were overfed either specifically during the far-off period or beginning during the far-off period and continuing until calving, suggest that overfeeding results in heightened insulin resistance during the peripartal period. This typically leads to lower dry matter intakes, increased mobilization of body condition score, increased circulating concentrations of nonesterified fatty acids (NEFA), and increased risk for metabolic disorders and poorer productive and reproductive performance. Based on Studies from Overton and collaborators 41 diet formulation should meet 110 to 120% of energy requirements during this period. For most Holsteins, this translates into targeted NEL intakes of 15 to 17 Mcal/day and energy densities of the far-off diet of no more than 1.30 to 1.35 Mcal/kg of NEL.
Given that high potassium content of forages is not a concern during the far-off period, a wider range of forage choices is acceptable for use in diets. Metabolizable protein supplies as calculated by CPM Dairy should be in the range of 1,000 grams/day, which typically dictates inclusion of supplemental protein sources in the diet along with a mineral and vitamin mix. During the close-up period, similar to the far-off period, energy intake well in excess of requirements appears to result in heightened insulin resistance and the metabolic cascade leading to lower feed intake postcalving, increased loss of body condition score, increased risk of metabolic disorders, and poorer productive and reproductive performance. Based on the studies of Overton and collaborators 41 energy intake of cows during the close-up period at 110 to 120% of energy requirements. For Holsteins, this is in the range of 16 to 18 Mcal/day of NEL. As a starting point, Overton 41 recommends to formulate the close-up diet at approximately 1.45 Mcal/kg of NEL if the group is a commingled cow/heifer group and approximately 1.38 to 1.40 Mcal/kg of NEL if the group is composed of mature animals and feed intake is high.
Protein and energy
Diets high in crude protein (17% to 19%) are typically fed during early lactation to both stimulate and support high milk production, however, high protein diets have been associated with Increased plasma or milk concentrations of urea nitrogen, ammonia, or both and thus, with reduced reproductive performance 6, 35, 42. Intake of high protein diets by lactating cows has been shown to alter the pH and the concentrations of other ions in uterine secretions, but only during the luteal phase and not at estrus. Uterine pH was also affected in heifers fed excess rumen degradable protein and was associated with reduced fertility 6. Plasma urea is inversely related to uterine luminal pH and sequential measurements in lactating cows have demonstrated that uterine pH is dynamically attuned to changes in plasma urea with a time lag of several hours 6.
As a result of feeding diets high in crude protein, increased plasma urea concentrations may interfere with the normal inductive actions of progesterone on the microenvironment of the uterus and, thereby, cause suboptimal conditions for support of embryo development 8. In vitro studies of bovine endometrial cell cultures have shown that urea alters both the pH gradient across the polarized cells and increases secretion of prostaglandin F2that may interfere with embryo development and viability 6. Recently Ocon and Hansen 40 demonstrated that embryo quality and development was reduced when embryos were cultured at a pH 7 or lower (uterine pH in vivo under high urea is 6.9). Moreover culture of bovine embryos with urea at the same concentration found in lactating cows reduced the proportion of oocytes becoming blastocyst probably due to a decreased competence to develop of the embryos formed by oocytes exposed to urea 40, 16.
The optimal dietary concentration of fermentable carbohydrates (i.e. fiber, sugar, and starch) is being refined for early lactation. Allen and collaborators 3 suggested that optimizing DMI requires different diets at different stages of the lactation because DMI is controlled by oxidation of fuels (fatty acids, propionate, lactate, and amino acids) in the liver in very early lactation and by gut fill as lactation proceeds towards its peak. Limiting dietary starch content and starch fermentability may increase DMI during the very early lactation period since there will be less rapid production and absorption of propionate 3. However, more fermentable carbohydrates (i.e. starch, non-forage fiber sources, and highly digestible forages) should be fed as lactation proceeds and plasma nonesterified fatty acids (NEFA) and -hydroxybutyrate (BHBA) decrease. Increasing the ruminal starch availability in a diet containing adequate physically effective NDF fed during the first 2 to 3 months may improve lactational and reproductive performance. Increasing the supply of glucogenic nutrients relative to lipogenic nutrients in early lactation may improve energy balance, decrease metabolic disorders, and improve reproduction through earlier resumption of the estrus cycle.
Gong and collaborators 24 showed that feeding a higher starch diet (26 vs. 10%) increased blood insulin concentration in early lactation and increased the proportion of cows that ovulated with the first 50 d postpartum. An increase in dietary glucogenic nutrients (27% starch) in grass and corn silage-based diets fed through 9 weeks postpartum improved energy status assessed by calculated energy balance, plasma NEFA and BHBA concentrations, and liver triglyceride content, but did not affect DMI or milk yield 52. Although increasing insulin by dietary manipulation can be beneficial for resumption of the estrous cycle, there is evidence that a high insulin status might have a detrimental effect on oocyte quality and embryo development 46.
Garnsworthy et al. 20 demonstrated that pregnancy rate was improved when a glucogenic diet that stimulated plasma insulin was fed before the first ovulation postpartum followed by a lipid-rich diet that lowered plasma insulin during the breeding period. Feeding a high starch (27%) diet for 50 d postpartum followed by a high fat (7%) diet until 120 d postpartum compared with an UK industry standard diet tended to increase the proportion of cows cycling by 50 d postpartum but did not affect conception rate 22. The diet switch was made at 50 d postpartum instead of at first ovulation so the higher starch diet may have been detrimental to embryo development. Caution is advised when formulating early lactation diets to stimulate the recrudescence of ovarian activity since highly fermentable starch diets fed immediately after calving may decrease DMI and prolong NEB 3.
Manipulation of the dietary fat content has been used in research and practice aiming to correct the energy balance defect and restore reproductive function. Different types of dietary fats were shown to have differential effect on reproductive function in cattle 55. Some fatty acids can be detrimental, as in the case of excessive negative energy balance in high yielding dairy cows, which results in an increased concentration of nonesterified fatty acids in serum and follicular fluid 36. In vitro studies showed that saturated fatty acids have toxics effect on granulosa cells 53 and have a negative effect on oocytes maturation, fertilization, cleavage and blastocyst yield 36. In contrast, polyunsaturated fatty acids (PUFAs) such as n-6 and n-3, positively affect fertility. In addition to their importance as energy source, PUFAs act as direct precursors of prostlaglandins 1. It was also suggested that PUFAs can differentially modulate the balance between production of prostaglandins E2 and F2 12, 14.
Rumen protected conjugated linoleic acid (CLA), in particular the isomers cis-9, trans-11 and trans-10, cis-12, have been shown to reduce NEFA and BHBA levels in blood 51, 38, 49, 10, to increase DMI and some of the negative acute phase proteins such as albumin and cholesterol 51, 19. Moreover, it has been demonstrated that CLA increase plasma IGF-I levels 10,19 thus it may stimulate the onset of the first ovulation after parturition. However in vitro studies conducted by Marei and collaborators 37 showed an adverse effect of CLA on oocytes maturation with a lower percentage of oocytes in metaphase II in oocytes cultured in CLA supplemented media compared to the control. More consistent results have been shown instead for the -linolenic acid (ALA) which it was proved to reduce early pregnancy loss 4 and increase oocytes cleavage rate 57. More recently, the effect of trans-fatty acid supplementation was observed in transition dairy cows 31. The results suggested an improvement in conception rate probably due to an increased fertilization rate of the oocytes and a better quality of embryos.
The Transition period involves multifactorial events. So the success in transition cow programs depends upon excellent management in a number of different areas. However, energy balance seems to be one of the most important factors affecting the fertility of transition dairy cows. Controlling energy and protein intake of cows during this phase is the key to prevent postpartum disorders and increase productivity of the herd.
1. Abayasekara, D.R.E. & Wathes, D.C. 1999, “Effects of altering dietary fatty acid composition on prostaglandin synthesis and fertility”, Prostaglandins, Leukotrienes and Essential Fatty Acids, vol. 61, no. 5, pp. 275-287.
2. Agenäs, S., Burstedt, E. & Holtenius, K. 2003, “Effects of feeding intensity during the dry period. 1. Feed intake, body weight, and milk production”, Journal of dairy science, vol. 86, no. 3, pp. 870-882.
3. Allen, M., Bradford, B. & Oba, M. 2009, “BOARD-INVITED REVIEW: The hepatic oxidation theory of the control of feed intake and its application to ruminants”, Journal of animal science, vol. 87, no. 10, pp. 3317-3334.
4. Ambrose, D., Kastelic, J., Corbett, R., Pitney, P., Petit, H., Small, J. & Zalkovic, P. 2006, “Lower Pregnancy Losses in Lactating Dairy Cows Fed a Diet Enriched in< i> </i>-Linolenic Acid”, Journal of dairy science, vol. 89, no. 8, pp. 3066-3074.
5. Berry, D., Buckley, F., Dillon, P., Evans, R., Rath, M. & Veerkamp, R. 2003, “Genetic relationships among body condition score, body weight, milk yield, and fertility in dairy cows”, Journal of dairy science, vol. 86, no. 6, pp. 2193-2204.
6. Butler, W.R. 1998, “Review: effect of protein nutrition on ovarian and uterine physiology in dairy cattle”, Journal of dairy science, vol. 81, no. 9, pp. 2533-2539.
7. Butler, S.T., Pelton, S.H. & Butler, W.R. 2006, “Energy balance, metabolic status, and the first postpartum ovarian follicle wave in cows administered propylene glycol”, Journal of dairy science, vol. 89, no. 8, pp. 2938-2951.
8. Butler, W.R. 2000, “Nutritional interactions with reproductive performance in dairy cattle”, Animal Reproduction Science, vol. 60-61, pp. 449-457.
9. Butler, W.R. & Smith, R.D. 1989, “Interrelationships between energy balance and postpartum reproductive function in dairy cattle”, Journal of dairy science, vol. 72, no. 3, pp. 767-783.
10. Castaneda-Gutierrez, E., Benefield, B.C., de Veth, M.J., Santos, N.R., Gilbert, R.O., Butler, W.R. & Bauman, D.E. 2007, “Evaluation of the mechanism of action of conjugated linoleic acid isomers on reproduction in dairy cows”, Journal of dairy science, vol. 90, no. 9, pp. 4253-4264.
11. Chagas, L., Bass, J., Blache, D., Burke, C., Kay, J., Lindsay, D., Lucy, M., Martin, G., Meier, S. & Rhodes, F. 2007, “Invited Review: New Perspectives on the Roles of Nutrition and Metabolic Priorities in the Subfertility of High-Producing Dairy Cows”, Journal of dairy science, vol. 90, no. 9, pp. 4022-4032.
12. Cheng, Z., Robinson, R.S., Pushpakumara, P.G., Mansbridge, R.J. & Wathes, D.C. 2001, “Effect of dietary polyunsaturated fatty acids on uterine prostaglandin synthesis in the cow”, Journal of Endocrinology, vol. 171, no. 3, pp. 463.
13. Collard, B., Boettcher, P., Dekkers, J., Petitclerc, D. & Schaeffer, L. 2000, “Relationships between energy balance and health traits of dairy cattle in early lactation”, Journal of dairy science, vol. 83, no. 11, pp. 2683-2690.
14. Coyne, G., Kenny, D., Childs, S., Sreenan, J. & Waters, S. 2008, “Dietary< i> n</i>-3 polyunsaturated fatty acids alter the expression of genes involved in prostaglandin biosynthesis in the bovine uterus”, Theriogenology, vol. 70, no. 5, pp. 772-782.
15. Dann, H., Litherland, N., Underwood, J., Bionaz, M., D’angelo, A., McFadden, J. & Drackley, J. 2006, “Diets during far-off and close-up dry periods affect periparturient metabolism and lactation in multiparous cows”, Journal of dairy science, vol. 89, no. 9, pp. 3563-3577.
16. De Wit, A., Cesar, M. & Kruip, T. 2001, “Effect of urea during in vitro maturation on nuclear maturation and embryo development of bovine cumulus-oocyte-complexes”, Journal of dairy science, vol. 84, no. 8, pp. 1800-1804.
17. Diskin, M., Mackey, D., Roche, J. & Sreenan, J. 2003, “Effects of nutrition and metabolic status on circulating hormones and ovarian follicle development in cattle”, Animal Reproduction Science, vol. 78, no. 3, pp. 345-370.
18. Douglas, G., Overton, T., Bateman II, H., Dann, H. & Drackley, J. 2006, “Prepartal plane of nutrition, regardless of dietary energy source, affects periparturient metabolism and dry matter intake in Holstein cows”, Journal of dairy science, vol. 89, no. 6, pp. 2141-2157.
19. Esposito, G., Absalôn Medina, V.A., Schneider, A., Gilbert, R.O. & Butler, W.R. 2012, Effect of dietary conjugated linoleic acid (CLA) on the metabolism and reproduction of dairy cows.
20. Garnsworthy, P., Fouladi-Nashta, A., Mann, G., Sinclair, K. & Webb, R. 2009, “Effect of dietary-induced changes in plasma insulin concentrations during the early post-partum period on pregnancy rate in dairy cows”, Reproduction, vol. 137, no. 4, pp. 759-768.
21. Gibb, M., Huckle, C., Nuthall, R. & Rook, A. 1997, “Effect of sward surface height on intake and grazing behaviour by lactating Holstein Friesian cows”, Grass and Forage Science, vol. 52, no. 3, pp. 309-321.
22. Gilmore, H.S., Young, F.J., Patterson, D.C., Wylie, A.R., Law, R.A., Kilpatrick, D.J., Elliott, C.T. & Mayne, C.S. 2011, “An evaluation of the effect of altering nutrition and nutritional strategies in early lactation on reproductive performance and estrous behavior of high-yielding Holstein-Friesian dairy cows”, Journal of dairy science, vol. 94, no. 7, pp. 3510-3526.
23. Goff, J.P. & Kimura, K. 2002, “Metabolic diseases and their effect on immune function and resistance to infectious disease”, National Mastitis Council Annual Meeting Proceedings, Orlando, FL, February, pp. 3.
24. Gong, J.G. 2002, “Influence of metabolic hormones and nutrition on ovarian follicle development in cattle: practical implications”, Domestic animal endocrinology, vol. 23, no. 1-2, pp. 229-241.
25. Hayirli, A., Grummer, R., Nordheim, E. & Crump, P. 2002, “Animal and dietary factors affecting feed intake during the prefresh transition period in Holsteins”, Journal of dairy science, vol. 85, no. 12, pp. 3430-3443.
26. Holtenius, K., Agenäs, S., Delavaud, C. & Chilliard, Y. 2003, “Effects of feeding intensity during the dry period. 2. Metabolic and hormonal responses”, Journal of dairy science, vol. 86, no. 3, pp. 883-891.
27. Horan, B., Dillon, P., Faverdin, P., Delaby, L., Buckley, F. & Rath, M. 2005a, “The interaction of strain of Holstein-Friesian cows and pasture-based feed systems on milk yield, body weight, and body condition score”, Journal of dairy science, vol. 88, no. 3, pp. 1231-1243.
28. Horan, B., Mee, J., Rath, M., O’connor, P. & Dillon, P. 2005b, “Effect of strain of Holstein-Friesian cow and feed system on reproductive performance in seasonal-calving milk production systems over four years”, XX International Grassland Congress: Offered PapersWageningen Academic Pub, , pp. 136.
29. Ingvartsen, K.L. & Andersen, J.B. 2000, “Integration of metabolism and intake regulation: a review focusing on periparturient animals”, Journal of dairy science, vol. 83, no. 7, pp. 1573-1597.
30. Jorritsma, R., Jorritsma, H., Schukken, Y.H. & Wentink, G.H. 2000, “Relationships between fatty liver and fertility and some periparturient diseases in commercial Dutch dairy herds”, Theriogenology, vol. 54, no. 7, pp. 1065-1074.
31. Juchem, S., Cerri, R., Villaseñor, M., Galvão, K., Bruno, R., Rutigliano, H., DePeters, E., Silvestre, F., Thatcher, W. & Santos, J. 2010, “Supplementation with Calcium Salts of Linoleic and trans-Octadecenoic Acids Improves Fertility of Lactating Dairy Cows”, Reproduction in Domestic Animals, vol. 45, no. 1, pp. 55-62.
32. Kennedy, J., Dillon, P., Delaby, L., Faverdin, P., Stakelum, G. & Rath, M. 2003, “Effect of genetic merit and concentrate supplementation on grass intake and milk production with Holstein Friesian dairy cows”, Journal of dairy science, vol. 86, no. 2, pp. 610-621.
33. Kolver, E. 2006, “Supplemental fumarate did not influence milksolids or methane production from dairy cows fed high quality pasture”, proceedings of New Zeland Society of Animal Producion. New Zealand Society of Animal Production; 1999, , pp. 409.
34. Kolver, E. & Muller, L. 1998, “Performance and nutrient intake of high producing Holstein cows consuming pasture or a total mixed ration”, Journal of dairy science, vol. 81, no. 5, pp. 1403-1411.
35. Lean, I.J., Celi, P., Raadsma, H., McNamara, J. & Rabiee, A.R. 2012, “Effects of dietary crude protein on fertility: Meta-analysis and meta-regression”, Animal Feed Science and Technology, vol. 171, no. 1, pp. 31-42.
36. Leroy, J., Vanholder, T., Mateusen, B., Christophe, A., Opsomer, G., de Kruif, A., Genicot, G. & Van Soom, A. 2005, “Non-esterified fatty acids in follicular fluid of dairy cows and their effect on developmental capacity of bovine oocytes in vitro”, Reproduction, vol. 130, no. 4, pp. 485-495.
37. Marei, W.F., Wathes, D.C. & Fouladi-Nashta, A.A. 2010, “Impact of linoleic acid on bovine oocyte maturation and embryo development”, Reproduction, vol. 139, no. 6, pp. 979-988.
38. Mattos, R., Staples, C.R. & Thatcher, W.W. 2000, “Effects of dietary fatty acids on reproduction in ruminants”, Reproduction, vol. 5, no. 1, pp. 38.
39. Mulligan, F., Dillon, P., Callan, J., Rath, M. & O’mara, F. 2004, “Supplementary concentrate type affects nitrogen excretion of grazing dairy cows”, Journal of dairy science, vol. 87, no. 10, pp. 3451-3460.
40. Ocon, O. & Hansen, P. 2003, “Disruption of bovine oocytes and preimplantation embryos by urea and acidic pH”, Journal of dairy science, vol. 86, no. 4, pp. 1194-1200.
41. Overton, T.R. 2012, Emerging concepts in metabolic regulation in transition dairy cows. Proceedings of the Advanced Dairy Nutrition Shortcourse
42. Rajala-Schultz, P., Saville, W., Frazer, G. & Wittum, T. 2001, “Association between milk urea nitrogen and fertility in Ohio dairy cows”, Journal of dairy science, vol. 84, no. 2, pp. 482-489.
43. Roche, J.F. 2006, “The effect of nutritional management of the dairy cow on reproductive efficiency”, Animal Reproduction Science, vol. 96, no. 3-4, pp. 282-296.
44. Rukkwamsuk, T., Wensing, T. & Geelen, M.J.H. 1999, “Effect of fatty liver on hepatic gluconeogenesis in periparturient dairy cows”, Journal of dairy science, vol. 82, no. 3, pp. 500-505.
45. Rukkwamsuk, T., Wensing, T. & Kruip, T.A.M. 1999, “Relationship between triacylglycerol concentration in the liver and first ovulation in postpartum dairy cows* 1”, Theriogenology, vol. 51, no. 6, pp. 1133-1142.
46. Santos, J., Bisinotto, R., Ribeiro, E., Lima, F., Greco, L., Staples, C. & Thatcher, W. 2010, “Applying nutrition and physiology to improve reproduction in dairy cattle”, Reproduction in Domestic Ruminants, vol. 7, no. 1, pp. 385-401.
47. Schneider, J.E. 2004, “Energy balance and reproduction”, Physiology & Behavior, vol. 81, no. 2, pp. 289-317.
48. Shah, M., Friedman, E., Bahaa, A. & Murphy, M. 2004, “Effect of liquid flavor supplementation of the diet on dairy cows in the transition period”, Journal of dairy science, vol. 87, no. 6, pp. 1872-1877.
49. Staples, C., Thatcher, W. & Clark, J. 1990, “Relationship Between Ovarian Activity and Energy Status During the Early Postpartum Period of High Producing Dairy Cows< sup> 1, 2</sup>”, Journal of dairy science, vol. 73, no. 4, pp. 938-947.
50. Steen, R. & Kilpatrick, D. 2000, “The effects of the ratio of grass silage to concentrates in the diet and restricted dry matter intake on the performance and carcass composition of beef cattle”, Livestock Production Science, vol. 62, no. 2, pp. 181-192.
51. Trevisi, E. & Bertoni, G. 2008, “Attenuation with acetylsalicylate treatments of inflammatory conditions in periparturient dairy cows”, Aspirin and health research progress.Nova Science Publ., Hauppauge, NY, USA, , pp. 23-37.
52. Van Knegsel, A., Van den Brand, H., Dijkstra, J. & Kemp, B. 2007, “Effects of dietary energy source on energy balance, metabolites and reproduction variables in dairy cows in early lactation”, Theriogenology, vol. 68, pp. S274-S280.
53. Vanholder, T., Leroy, J., Soom, A.V., Opsomer, G., Maes, D., Coryn, M. & Kruif, A.d. 2005, “Effect of non-esterified fatty acids on bovine granulosa cell steroidogenesis and proliferation in vitro”, Animal Reproduction Science, vol. 87, no. 1, pp. 33-44.
54. Villa-Godoy, A., Hughes, T., Emery, R., Chapin, L. & Fogwell, R. 1988, “Association between energy balance and luteal function in lactating dairy cows”, Journal of dairy science, vol. 71, no. 4, pp. 1063-1072.
55. Wathes, D.C., Cheng, Z., Chowdhury, W., Fenwick, M.A., Fitzpatrick, R., Morris, D.G., Patton, J. & Murphy, J.J. 2009, “Negative energy balance alters global gene expression and immune responses in the uterus of postpartum dairy cows”, Physiological genomics, vol. 39, no. 1, pp. 1-13.
56. Wathes, D., Cheng, Z., Bourne, N., Taylor, V., Coffey, M. & Brotherstone, S. 2007, “Differences between primiparous and multiparous dairy cows in the inter-relationships between metabolic traits, milk yield and body condition score in the periparturient period”, Domestic animal endocrinology, vol. 33, no. 2, pp. 203-225.
57. Zachut, M., Arieli, A., Lehrer, H., Livshitz, L., Yakoby, S. & Moallem, U. 2010, “Effects of increased supplementation of n-3 fatty acids to transition dairy cows on performance and fatty acid profile in plasma, adipose tissue, and milk fat”, Journal of dairy science, vol. 93, no. 12, pp. 5877-5889.